Epoxides such as ethylene oxide, propylene oxide, 1,2-butene oxide and the like are useful intermediates for the preparation of a wide variety of products. The oxirane functionality in such compounds is highly reactive and may be ring-opened with any number of nucleophilic reactants. For example, epoxides may be hydrolyzed to yield glycols useful as anti-freeze components, food additives, or reactive monomers for the preparation of condensation polymers such as polyesters.
Polyether polyols generated by the ring-opening polymerization of epoxides are widely utilized as intermediates in the preparation of polyurethane foams, elastomers, sealants, coatings, and the like. The reaction of epoxides with alcohols provides glycol ethers, which may be used as polar solvents in a number of applications.
Many different methods for the preparation of epoxides have been developed. One such method involves the epoxidation of an olefin in a liquid phase reaction using an organic hydroperoxide as the oxidizing agent and certain transition metal compounds as catalyst. Generally speaking, Group IV, V, and VI transition metal compounds have been found to have the highest activity and selectivity in olefin epoxidation reactions using organic hydroperoxides. Metals having low oxidation potentials and high Lewis acidity in their highest oxidation states are superior epoxidation catalysts, according to Sheldon, J. Mol Cat. 7, 107(1980). Molybdenum, tungsten, titanium, and vanadium compounds thus have been found to be the most useful catalysts for the reaction of an organic hydroperoxide with an olefin.
Sheldon found that rhenium heptoxide, in contrast to other transition metal compounds, caused rapid, non-productive decomposition of the hydroperoxide. Thus, an attempt to epoxidize 1-octene with t-butyl hydroperoxide using rhenium heptoxide gave complete conversion of the hydroperoxide to the corresponding alcohol but none of the desired epoxide product. Kollar [U.S. Pat. No. 3,351,635 (Table I); Preprints, Dev. Pet. Chem. 106(1978)] also reported extremely low yields of epoxide when the use of a rhenium catalyst in an epoxidation reaction was attempted, apparently due to the very rapid decomposition of the hydroperoxide catalyzed by the rhenium compound. Low selectivity to epoxide was similarly observed using rhenium decacarbonyl as catalyst, cyclohexene as the olefin substrate, and t-butyl hydroperoxide as the oxidant [Rummel et al. Oxid. Commun. 6, 319(1984)]. Jorgensen, in a recent review of transition metal catalyzed epoxidations [Chem. Rev. 89, 431(1989)], concludes that rhenium complexes are poor epoxidation catalysts using t-butyl hydroperoxide as oxidant.
To date, rhenium compounds such as rhenium heptoxide have thus been primarily used to promote decomposition reactions of hydroperoxides rather than as oxidation catalysts. For example, European Pat. Appl No. 308,101 employed rhenium compounds to catalyze the decomposition of t-butyl hydroperoxide to t-butyl alcohol. Jpn. Kokai No. 63-277,640 (Chem Abst 110:172753d) teaches cyclohexyl hydroperoxide decomposition to cyclohexanol or cyclohexanone using rhenium heptoxide. U.S. Pat. No. 4,297,518 describes the use of rhenium heptoxide to catalyze the rearrangement of cumene hydroperoxide to phenol and acetone.
It is thus apparent that rhenium compounds have heretofore been found to be of little utility as catalysts for the epoxidation of olefins using alkyl hydroperoxides as the source of oxygen, owing to the tendency of such compounds to favor hydroperoxide decomposition over olefin epoxidation. We have now found that certain rhenium porphyrin complexes, in contrast to the rhenium compounds employed in the prior art, are excellent olefin epoxidation catalysts and permit the preparation of epoxides with high selectivity and minimal unproductive hydroperoxide decomposition.